Thermal conversion processes, such as coking, are commonly used in petroleum refineries for converting heavy hydrocarbon feedstocks to more valuable lower boiling products. Examples of two types of coking effective for this invention are short vapor contact time coking and fluidized bed coking. Short vapor contact time coking contains a short vapor contact time reaction zone containing a horizontal moving bed of fluidized hot solids recycled from a heating zone. The reaction zone is operated at a temperature from about 450° C. to about 700° C. (842–1292° F.) and under conditions such that the solids residence time and the vapor residence time are independently controlled. Conventional fluidized bed coking process units typically include a coking zone, a stripping zone, a coke regeneration zone and overhead equipment. A heavy carbonaceous petroleum feedstock is introduced into the coking zone containing a fluidized bed of hot solids, preferably coke. The feedstock is distributed as uniformly as possible over the surfaces of said coke particles where it is cracked to vapors and carbonaceous material that is deposited onto the hot solids. The vapors pass through cyclones that remove entrained coke particles. The vapor is then discharged into a scrubbing zone where any remaining solid particles are removed, the heaviest product is condensed and the vapor is then cooled to condensed products, which go to the fractionator. A slurry of heavy liquid and solid particles, which usually contains from about 1 to about 3 wt. % coke particles, is recycled to extinction from the scrubber to the coking zone.
During the coking process, the feedstocks that are thermally cracked have a tendency to form carbonaceous, insoluble solid deposits that coat and plug process equipment. The deposition of these deposits on process equipment is called fouling and the deposits are called the foulant. Coke plugs lines and damages overhead equipment such as cyclones. Even small amounts of coke deposited on the surface of process equipment can greatly reduce the efficiency of the equipment by reducing heat transfer. Large amounts of coke deposited on the surface of process equipment, e.g. cyclone snouts, can result in high-pressure drop which reduces throughput such that the unit has to be shut down to remove the coke deposits. Fouling may be due to a variety of causes, including feedstock entrainment and condensation of vaporized feedstock on surfaces that subsequently undergo thermal conversion to coke. Unfortunately, when such fouling occurred, one could not differentiate whether the source of the fouling was due to feedstock entrainment of small feed droplets or condensation of vaporized feedstocks. Therefore, there is a need in the art for a method for determining the source of such fouling so that the process conditions or the overhead equipment can be adjusted to reduce and/or mitigate coking.